In September, I will reach the end of my second term as the first national leader of the Green party of England and Wales – and I've decided not to seek re-election for another two-year term.

When I first joined the party back in 1986, it was a core article of faith for many members that we should not have a conventional leader in the traditional mould. It's true that formal leadership carries risks. Presenting one image as the "face" of a party can attract some people, but put off others. At a deeper level, if leadership is about concentrating power in the hands of a single person or an inner circle, it is damaging to the cause and can lead to poor decision-making.

But leadership is also a powerful tool that can draw people in and inspire them. Trying to sell abstract ideas to the public rarely works. People want to see the human face of an organisation, to help them understand what it is there for and to judge if they trust it. So when the decision was finally made in 2007 to establish the roles of leader and deputy leader of the Green party, it wasn't about sacrificing a principle to gain some votes. It was about changing the way we worked to help communicate our passion and principles in a more effective way. It is an example of how we found the right balance between principles and pragmatism, and how the party had become more self-confident.

It was always my view that leadership, for the Greens, did not have to be about seizing power and holding on to it at all costs. Leaders must have confidence in their abilities as well as their cause. But to ensure that every individual in the movement feels a personal responsibility, they should also be eager to share out the responsibilities of leadership with others, from shaping policy to maintaining morale. In other Green parties around the world, leaders have often been the first to recognise that the time has come for others to take up that responsibility; a healthy contrast to seeing traditional politicians clawing at their desks in a vain attempt to hang on to power.

I am hugely honoured to have served my party in this role, and proud that during the four years of my term, we've moved Green politics forward to a higher level, with the party by far the most influential it has ever been. We have seen significant breakthroughs, not only in Brighton & Hove, winning our first seat at Westminster and our first ever local council, but also nationally, with further breakthroughs on to new councils in the recent local elections, as well as establishing ourselves as the third party, ahead of the Liberal Democrats, in the elections for London mayor and the London Assembly.

Green politics has shifted from the margins to the mainstream of British political life. As the party grows, buoyed by a new generation of activists and underpinned by the achievements of our outstanding elected members, now is the time to make space for other talented Greens to come through and take us even further forward. For me, this decision is about staying true to green principles. We're lucky to have a wealth of capability and experience in our ranks, and taking a strategic approach to leadership is a natural step in maturing as a party and nurturing future electoral aspirations. I'm confident that the next leader, whoever they may be, will be able to build on our momentum.

Now I look forward to channelling even more of my energy into representing the people of Brighton Pavilion, speaking out in parliament on behalf of my constituents and doing all I can to defend them against the coalition government's disastrous economic policies. I will continue to be a lead advocate for the party, and will hold the government to account on its environmental and social responsibilities.

The Greens have a clear vision of how we must now move away from our broken economic system – which for too long has kept us dangerously dependent on the bankers' gambles, toxic debt and on the plundering of natural resources – and towards a fairer, more sustainable economy. People are increasingly recognising that we are a credible alternative to the three main Westminster parties, and as communities up and down the country feel the consequences of savage yet self-defeating austerity – paying the price for a crisis they did not create – that Green alternative has never been more necessary.